1. Field of the Invention
This invention relates to a color television (TV) camera, and more particularly to a color resolving prism system positioned between an objective lens and the focal plane of the objective lens for color-resolving the light beam from an object to be photographed.
2. Description of the Prior Art
In a color TV camera, a color resolving prism system is disposed behind an objective lens, and the incident light beam is resolved into three color wavelength ranges and imaged on three image pickup elements corresponding to the respective wavelength ranges. Each image pickup element scans on the imaging plane and converts the image into an electrical signal.
Light weight and compactness are required as much as possible of these color TV cameras, particularly hand-held color TV cameras for collecting news, and therefore, it has been practiced to make the image size small and make the color resolving prism system and image pickup elements compact.
For example, the image pickup tube converts the energy of light arriving at the light-receiving surface into an electric current by scanning of an electron beam and obtains an output signal. Accordingly, when the image size is made small and an objective lens of the same F-number is used, if the same object to be photographed is photographed, the energy of light per unit area on the light-receiving surface is the same, but the cross-sectional area of the beam becomes smaller in proportion to the image size and therefore the output current decreases. This means that sensitivity is reduced if the image size is made smaller. Accordingly, to avoid a reduction in sensitivity, it is necessary to make the F-number of the objective lens smaller with the reduction in the size of the picture plane and make the energy of light arriving at the light-receiving surface great to thereby eliminate any reduction in the output current.
However, in a color resolving prism comprised of three prisms, an F-number of about 1.4 has been the limit and it has been difficult to make the aperture opening great.
Problems will first be illustrated by reference to an example of the prior art shown in FIG. 1 of the accompanying drawings. The imaged light beam emerging from an objective lens 4 enters a first prism from the entrance surface 1' of the color resolving prism system and for example, only the blue range light thereof is reflected by a surface 1" provided with a dichroic film and is further totally reflected by the entrance surface 1', whereafter the unnecessary wavelength component thereof is cut by a trimming filter 6B, and then the light beam is imaged on the light-receiving surface 5B' of an image pickup element 5B.
The light beam transmitted through the dichroic surface 1" enters a second prism 2 and for example, only the red range light thereof is reflected by a surface 2" provided with a dichroic film and is further totally reflected by the boundary surface 2' with a parallel air gap provided between the first prism 1 and the second prism 2, and the unnecessary wavelength component thereof is cut by a trimming filter 6R, and then the light beam is imaged on the light-receiving surface 5R' of an image pickup element 5R. The light beam transmitted through the dichroic surface 2", for example, the green range light, passes through a prism 3 and the unnecessary wavelength component thereof is cut by a trimming filter 6G, and then the light beam is imaged on the light-receiving surface 5G' of an image pickup element 5G. The shapes of the prisms are determined by the specifications such as the refractive index n of the glass material and the F-number (F.sub.no) desired. If, as shown in FIG. 1, the angles formed between the light entrance surface 1' of the first prism 1 and the dichroic surface 1", and between the light entrance surface 2' of the second prism 2 and the dichroic surface 2" are .theta..sub.1 and .theta..sub.2 and the angle formed between the light entrance surface of the third prism system 3 and the light exit surface 3" of the thrid prism system 3 is .theta..sub.3, these angles must satisfy the following conditions: EQU .theta..sub.1 .ltoreq.sin.sup.-1 (1/n)-sin.sup.-1 (1/(2nF.sub.no)) (3) EQU 2.theta..sub.1 .gtoreq.sin.sup.-1 (1/n)+sin.sup.-1 (1/(2nF.sub.no)) (4) EQU 2.theta..sub.2 .gtoreq.0.sup.1 +sin.sup.-1 (1/n)+sin.sup.-1 (1/(2nF.sub.no)) (5) EQU .theta..sub.3 =.theta..sub.2 -.theta..sub.1 ( 6)
Condition (3) is necessary in order that the wavelength range light to be transmitted through the dichroic surface 1" may not be totally reflected by the surface 1", condition (4) is necessary in order that the wavelength range light reflected by the dichroic surface 1" may be totally reflected by the surface 1', condition (5) is necessary in order that the wavelength range light reflected by the dichroic surface 2" may be totally reflected by the surface 2', and condition (6) is necessary in order that the entrance surface 1' and the exit surface 3" may be parallel to each other.
It is to be noted that the angle formed between the light entrance surface of the first prism 1 and the dichroic surface, i.e., the angle .theta..sub.1 formed between a plane perpendicular to the optic axis and the dichroic surface 1", the range in which the angle .theta..sub.1 can exist is determined by the refractive index n of the glass material and the F-number desired in accordance with conditions (3) and (4).
FIG. 2 of the accompanying drawings shows this fact and also shows the relation between the F-number and the angle .theta. with the refractive index n of the glass material as the parameter. It can be seen from this graph that the range of the angle .theta..sub.1 which satisfies conditions (3) and (4) at a time is limited to a range in which the F-number is greater than 1.4, irrespective of the refractive index n of the glass material. That is, in a color resolving prism system comprised of three prisms, an F-number of 1.4 is the limit, whereby even if a bright lens having a wide aperture opening is used, regular reflection and total reflection do not take place and therefore a predetermined color resolving action is not executed.
As described above, in a color resolving prism of the conventional type, only objective lenses of up to F-number 1.4 can be used, and this has led to a disadvantage that reduction in sensitivity is unavoidable if the image is made smaller and the camera is made compact.
In contrast, a method of alleviating the limit of the brightness of a color resolving prism system comprised of three prisms is announced in "New Camera Technology and Digital Technique, Television Technology in the 80's".
That is, in the conventional color resolving prism system as is shown in FIG. 1, the entrance surface of the first prism is inclined by an angle .theta..sub.10 in the direction of a side opposite to the dichroic surface 1" and to make the entrance surface and the exit surface 3" of the color resolving prism system parallel to each other, a wedge-shaped prism 10 having a refracting angle .theta..sub.10 is provided in front of the first prism 1 with a parallel air space interposed therebetween, and thus the system of FIG. 3 is attained. In this case, the aforementioned condition (4) is modified as follows: EQU 2.theta..sub.11 +.theta..sub.10 .gtoreq.sin.sup.-1 (1/n)+sin.sup.-1 (1/(2nF.sub.no)).thrfore.2.theta..sub.11 .gtoreq.sin.sup.-1 (1/n)+sin.sup.-1 (1/(2nF.sub.no)) (4)'
This corresponds to the fact that curve (4) in FIG. 2 is lowered by .theta..sub.10 /.sub.2 to become curve (4') and the point of intersection between curve (4) and curve (3) moves to the left, i.e., in the direction in which the F-number becomes smaller. In this novel color resolving optical system comprising four prisms, a color resolving prism in which an objective lens whose F-number is smaller than 1.4, for example, F-number 1.2, is usable.
U.S. Pat. Nos. 4,236,177 and 4,262,305 propose a color resolving prism system using four prisms, but the F-number of the specific embodiments described therein is 1.4.
As a result of the inventor's study, it has been found that the arrangement of this color resolving prism system suffers from problems such as an increase in the optical path length of the glass caused by an increase in the number of constituent prisms, bulkiness of the entrance surface resulting from making the F-number smaller, occurrence of a ghost image caused by an increase in the number of surfaces, etc.